Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistance variable memory, such as phase change random access memory (PCRAM) and resistive random access memory (RRAM), among others.
A physical layout of a PCRAM memory device may resemble that of a DRAM device, with the capacitor of the DRAM cell being replaced by a phase change material, such as Germanium-Antimony-Telluride (GST), which may be coupled to an access device, such as a diode, a field effect transistor (FET), or a bipolar junction transistor (BJT), for example.
The phase change material of a PCRAM device may exist in an amorphous, higher resistance state, or a crystalline, lower resistance state. The resistance state of the PCRAM cell may be altered by applying sources of energy to the cell, such as current pulses or pulses of light, among other sources of energy. For example, the resistance state of the PCRAM cell may be altered by heating the cell with a programming current. This results in the PCRAM cell being programmed to a particular resistance state, which may correspond to a data state. In a binary system, for example, the amorphous, higher resistance state may correspond to a data state of 1, and the crystalline, lower resistance state may correspond to a data state of 0. However, the choice of these corresponding data states may be reversed, that is, in other binary systems, the amorphous, higher resistance state may correspond to a data state of 0, and the crystalline, lower resistance state may correspond to a data state of 1. PCRAM devices may also be configured to provide multi-level storage. That is, the memory device may have a plurality of discrete and identifiable states which allow for multi-bit storage in a single memory cell.
Various failure and degradation mechanisms of PCRAM cells are associated with the interface between the phase change material and surrounding materials (e.g., between the phase change material and electrodes and/or interconnects). For example, heat loss due to heat transfer to adjacent cells and/or adjacent materials can result in increased reset currents. PCRAM cells can also experience degradation over time due to atomic migration between the phase change material and electrode material, which can result in poor cycling endurance, for instance.
As such, some previous PCRAM architectures include attempts to control the geometry or physical dimensions of PCRAM cells in order to isolate the active region away from the electrode interfaces. Examples of such previous architectures include, various PCRAM bridge cell structures and confined cell structures. However, such previous architectures can have various drawbacks such as limited scalability and/or complicated fabrication processes and may not provide sufficient thermal properties (e.g., heat loss characteristics) associated with operation of the PCRAM cells.